Seedless table grapes are a commercially attractive commodity with high consumer demand. A common problem encountered in the production of these grapes is that they tend to grow as small berries in compact clusters, which results in berry rupture and bunch rot. Thinning of these clusters is necessary in order to reduce the occurrence of bunch rot and to allow the berries which remain an opportunity to increase in size. Both of these factors are important in terms of producing a commercially acceptable crop.
Several strains of seedless grapes exist, such as Thompson Seedless and Perlette Seedless. The Perlette Seedless table grape strain is an early-maturing variety. The mature fruit have a striking translucence, thus the name "Perlette" signifying "little pearl". The fruit clusters are distinct in the compactness and earliness of maturity. This latter aspect is the primary reason why Perlettes are of commercial significance. Perlette clusters are large, about 450 to 700 grams, and very compact. The spherical berries are covered with an abundant waxy bloom and have a pale gold color. The clusters tend to be overly compact and subject to rotting unless extensive thinning of the berries is undertaken. Without thinning, the mature berries are undersized and not commercially acceptable.
There is currently no chemical treatment that is commercially used to thin Perlette Seedless grape clusters. Thinning of Perlette Seedless grapes currently requires extensive hand-thinning, which is time consuming and expensive.
Gibberellins are chemicals that are naturally found in small amounts in all plant species, including grapes. They function as regulators of plant growth. Giberellins are obtained commercially by growing large cultures of Gibberella fungus in nutrient medium and then isolating the gibberellins from the liquid medium.
One of the principal effects of gibberellins on plants is to promote rapid growth and elongation of cells that are still growing or are capable of growth. Gibberellic acid has also been shown to increase fructification of fruit trees. Belgian Patent No. 756990. When applied on grapes post-bloom, these compounds can produce larger berries.
The use of surfactants is commonplace for many agricultural applications. Surfactants modify the physical:chemical characteristics of agricultural formulations. These modifications include: lower surface tensions, emulsification of formulation components, and increased wetting/retention of spray droplets on plant surfaces. Behrens, R. W., Weeds, (1964) 12:255-258.
While all surfactants are capable of the physical:chemical modifications described above, only a few surfactants also demonstrate biological activity in plant tissues. Parr et al. (1965) Bot. Gaz. 126:86-96. This tends to complicate the process of selecting the right surfactant for a particular use because the relationship between biological activity and surfactant chemistry/structure is not well understood. Foy, C. L., Adjuvants and Aqrochemicals, Vol. I; Chow et al. (1989), CRC Press, pp. 1-15; Matsui et al. (1989) Proc. PGRSA Ann. Mtg. pp 234-236. As it stands now, there is no way to accurately predict the biological activity of a surfactant in a given plant system.
Gibberellins have successfully been utilized to enhance berry size and reduce berry set in seedless grape cultures, particularly Thompson Seedless. Lynn et al, Am. J. Enol. Viticult., 17:283-289 (1966); Dass et al., J. Hort. Sci., 52:189-191 (1977). This is an accepted commercial practice.
The application of gibberellin together with a modified phthalic glycerol alkyd resin surfactant (Triton B-1956; Rohm and Haas Co., Philadelphia, Pa.) has been used effectively to thin clusters of Thompson Seedless grapes. Similar treatments with a C.sub.10 to C.sub.18 linear polyethoxylated alcohol surfactants have been carried out on a seedless variety of Kishmish black grapes utilizing gibberellin. Salenkov et al. (1984) Puti Intensif. Vinocrad., pp. 66-71 (Puponin, A. E., ed., Mosk. S-kh. Akad.:Moscow, USSR). The gibberellin/surfactant composition was applied post-bloom and resulted in an increased yield of Kishmish grapes.
The application of gibberellic acid and a modified phthalic glycerol alkyd resin surfactant (e.g., Triton B-1956) to Perlette Seedless grape clusters has a variable effect on thinning, frequently demonstrating little or no significant thinning activity. The composition was applied to the Perlette Seedless grapes in California during bloom, and only minimal thinning was obtained. Kasimatis et al. Am. J. Enol. Vitic., 22:19-23 (1971).
Several studies have been conducted in India on the effect of gibberellins upon thinning of Perlette Seedless grapes. The results of these studies have shown mixed effects. The application of gibberellic acid alone (i.e., no surfactant included) to Perlette Seedless grapes produced thinning as the gibberellic acid concentration increased, but the percentage of "shot" berries (i.e., berries which remain small and do not mature physiologically over the course of the growing season) also increased. Nijjar et al., Indian J. Hort., 28:199-202 (1971); Dhillon et al., J. Res. Punjab Agric. Univ., 10:331-336 (1973). The presence of "shot" berries in a grape cluster is commercially unacceptable.
When gibberellic acid was combined with "soap" and applied to Perlette Seedless grape clusters, larger, heavierberries with a higher sugar/acid ratio were produced. Tripathi, Punjab Hort. J., 8:60-64 (1967). The combination of gibberellic acid with dish soap (Teepol, National Organic Chemical Industries, Ltd., Bombay, India) and subsequent application to Perlette Seedless grapes during pre-bloom produced significant thinning of grape clusters; however, the percentage of "shot" berries doubled. Singh et al., South Indian Hort., 33:330-332 (1985). In another study with combinations of gibberellic acid (at four different concentrations) and Teepol applied to Perlette Seedless grape clusters, three out of four treatments, including the lowest and highest gibberellic acid concentrations tested, showed no significant thinning activity. Sandhu et al., Harvana J. Hort. Soc., 14:221-214 (1985). Application of gibberellic acid together with a surfactant (Triton) as a wetting agent significantly reduced the compactness of berries in Perlette Seedless grape clusters; however the percentage of "shot" berries tended to increase. Chaturvedi et al., Vitis, 18:10-16 (1979).
Because of this variable effect on thinning activity and "shot" berry development, gibberellic acid formulations are not currently used for commercial thinning of Perlette grapes. Instead, hand-thinning is used in commercial practice.